The present invention refers to foam sheet laminates, such as those used for instance for packaging, construction, and sports, with good flexural and compression strength and resistance to bending.
Foamed products, which find use as packaging, cushioning, insulating and structural materials, typically consist of a phase of open or closed pores or cells dispersed throughout a polymer matrix. A wide array of processes have been devised for developing the cell phase in these products, including adding a gaseous xe2x80x9cblowing agentxe2x80x9d to the polymer during processing, producing a gaseous blowing agent by chemical reaction within the polymer during processing and forming the product from polymer granules to obtain a cellular structure. In one particularly popular process, a gaseous blowing agent is incorporated into a molten thermoplastic material to form a mixture which may then be molded to a desired shape, such as by extrusion. After molding, applied heat and/or reduced pressure causes the blowing agent to expand, forming a cellular structure within the thermoplastic matrix. The effectiveness of a particular blowing agent will depend largely upon the polymer composition in which it is incorporated, the method of incorporation, the process conditions, the additives used, and the products sought.
Blowing agents work by expanding the polymer to produce a cellular structure having far less density than the polymer itself. In processes in which a blowing agent is incorporated into a molten thermoplastic polymer, bubbles of gas form around xe2x80x9cnucleation sitesxe2x80x9d and are expanded by heat or reduced pressure. A nucleation site is a small particle or a conglomerate of small particles that promotes the formation of a gas bubble in the polymer. Additives may be incorporated into the polymer to promote nucleation for a particular blowing agent and, consequently, a more uniform pore distribution.
Once bubbles of the blowing agent have expanded to form the cellular structure, the structure is maintained by replacing the blowing agent in the cells with air. Diffusivity of the blowing agent out of the cells relative to air coming into the cells impacts the stability of the foam over time and whether the cells of the foam may collapse. Additives may be incorporated into the polymer and process conditions may be adjusted to assist in controlling the diffusivity of the blowing agent, to promote foam stability, and to limit collapse of the foam to acceptable limits.
Many methods are available for adding a blowing agent to a polymer during processing to produce a foam. In one method pertinent to the present invention, the blowing agent is mixed with a molten thermoplastic polymer under pressure, and the mixture is then extruded through a forming die of a desired configuration. Plank, which can be cut to a desired shape and thin foam sheets may also be produced in this manner.
Prior art processes for forming expanded foam products from thermoplastic polymers, typically used halogenated hydrocarbons as blowing agents. The halogenated hydrocarbons include the chlorofluorocarbons (xe2x80x9cCFCsxe2x80x9d) and hydrochlorofluorocarbons (xe2x80x9cHCFCsxe2x80x9d). CFCs and HCFCs are readily impregnable in thermoplastic polymers and are readily expandable under relatively mild conditions. CFCs and HCFCs generally produce foams of high quality with a minimum of processing difficulty. The pore size is controllable, the foam has good stability with minimum tendency to collapse after a period of time, and the surface characteristics of the foam are smooth and desirable. Also, CFCs, HCFCs and other halogenated hydrocarbons typically are either not flammable or are of low flammability, which greatly reduces the care with which they may be used. These compounds have the further advantage of low toxicity. However CFCs, HCFCs and other halogenated hydrocarbons have been linked to ozone depletion in the atmosphere. As a result of concern over the ozone layer, the use of these materials is being phased out in favour of materials which are more friendly to the ozone layer, such as hydrocarbons.
Although hydrocarbons are readily available, inexpensive and very compatible with polyethylene and other polymer matrix materials, thereby permitting wide processing variability, they present their own unique problems. Foremost among these problems is the greater flammability of these materials. Other problems with hydrocarbon blowing agents may include toxicity or environmental incompatibility. Moreover, the hydrocarbon blowing agents are slow to permeate through the expanded foam structure, such that the flammability and other problems associated with these materials persist in the foam structures for longer periods of time. Safety concerns have therefore mandated that manufacturers of foam structures store them for excessively long periods of time to enable the blowing agents therein to dissipate to levels below their lowest explosive limit so that the products are safe enough to be shipped to and used by customers.
A solution to the problem associated with the use of hydrocarbon blowing agents has been provided in the past by perforating the foam plank with a multiplicity of channels extending from one surface of the structure to the opposite surface.
Perforated foam plank have been commercialized world-wide by the present applicant in the eighties and a method for providing accelerated release of a blowing agent from a plastic foam by perforating it to form channels therein free of direction with respect to the longitudinal extension of the foam, has been described in U.S. Pat. No. 5,424,016.
The presence of these channels through the entire thickness of the plank however decreases the mechanical properties of the expanded foam plank, including its compression strength, resistance to creep, cushioning ability and the like. Furthermore a perforated foam plank can only be employed in those applications where impermeability is not required as water drops and other liquids can easily penetrate through the open pores on both plank surfaces. Another disadvantage of this method resides in the poorly attractive appearance of the plank where pores open on the surfaces.
U.S. Pat. No. 5,776,390 describes perforating an expanded foam sheet by a multiplicity of channels extending from one surface of the sheet to about 60 to 97% of the sheet thickness. Said perforation, hereinafter referred to as xe2x80x9cpartial perforationxe2x80x9d, leads to a foam sheet wherein one of the surfaces does not have any holes thereon (xe2x80x9cnon perforated surfacexe2x80x9d) while the other has the desired pattern of holes appearing thereon (xe2x80x9cperforated surfacexe2x80x9d). The problem with the approach described in U.S. Pat. No. 5,776,390 is that in order to prevent residual stresses to develop in the foam and thus decrease the sheet mechanical properties, perforation needs to be carried out when the extruded foam plank has been stabilized, what generally occurs about one hour after the extrusion. This means that perforation needs to be carried out off-line if foam warpage has to be controlled. Furthermore the use of a partially perforated foam plank highly restricts the applications thereof as at least one of the plank surfaces is not impermeable.
Despite the efforts that have been made in the past, there remains a need for an expanded foam plank that can be obtained by an industrially acceptable production method which accelerates the removal of a majority of the blowing agent from the expanded foam structure and maintains impermeability, good mechanical properties, and an attractive appearance.
The present invention addresses these needs.
The present invention refers to a foam plank laminate of thermoplastic material that comprise at least two foam sheets, both partially perforated, and laminated with the perforated surfaces facing each other and the smooth non perforated surfaces as the outer plank surfaces.
It has now been found in fact that if a plank of a given thickness is obtained by laminating at least two pre-formed, partially perforated, foam sheets in such a way that the non perforated smooth surfaces thereof become the plank outer surfaces, the thus obtained laminate, besides being impermeable and therefore suitable for any type of applications, has mechanical properties that are almost comparable to those of a non perforated plank of the same overall thickness and better than those of a perforated plank of the same overall thickness. It also been found that in order to achieve these mechanical properties in the final plank laminate it is not necessary to wait until the expanded foam of the forming sheets has been completely stabilized but that perforation of the foam sheets to be laminated according to the present invention can easily be made in-line, soon after extrusion, thus highly simplifying the manufacturing process.
A first object of the present invention is therefore an expanded foam plank laminate of thermoplastic material comprising a first partially perforated expanded foam sheet having a first surface which is not perforated and a second surface which is perforated, and a second partially perforated expanded foam sheet having a first surface which is non perforated and a second surface which is perforated, wherein the non perforated surfaces of the above forming sheets are the outer surfaces of the end expanded foam plank.
In one embodiment of the present invention the expanded foam plank laminate consists of only two partially perforated foam sheets that are laminated with the respective perforated surfaces directly adhered one to the other, leaving the respective non perforated surfaces as the foam plank laminate outer surfaces.
In another embodiment the foam plank laminate comprises more than two foam sheets, wherein one or more foam sheets are positioned between the first and the second partially perforated foam sheets and the non perforated respective surfaces of said first and second foam sheets are the foam plank laminate outer surfaces. In such a case the inner foam sheets can be perforated, either partially or completely, or non perforated. Preferably however they are at least partially perforated.